I was lucky enough to breakthrough on my first dose, and the only times I haven't broken through have been down to choice (to feel what lower doses felt like) – I invested in better equipment (http://imgur.com/vjvKzQB – from one of my posts in the /r/DMT channel in reddit) after the first few breakthroughs, it was the most interesting, terrifying, and beautiful thing that has ever happened to me, and I wanted to keep doing it to try and work out just what it was, there have been moments where I have lay in awe completely convinced I am witnessing everything happening everywhere in the universe, there have been times where I've been convinced I am solely responsible for the destruction of the universe, that my consciousness was the last thing left, there have been so many other strange experiences, I could never control them, I had to just sit back and feel everything, so it was always trying, I doubt that it's possible to master the effects, and most I know from various communities on the subject tend to ease off at around the same number of times that I did, as a chemical, it's absolutely non-addictive, all psychedelics I tend to find as self-regulating.